The present invention relates to a digital audio broadcasting receiver in which each carrier is subjected to differential phase modulation and orthogonal frequency division multiplexing (OFDM).
As a system which permits transmission of digital data to a mobile object which is strongly affected by the problems of radio wave propagation, such as the multipath and fading, the orthogonal frequency division multiplexing (OFDM) transmission system is known, and the use of this system in broadcasting is under way. Its typical example is seen in digital audio broadcasting (DAB) which is set forth in ITU-R Recommendation BS.774.
FIG. 16 is a block diagram of a digital audio broadcasting receiver.
In the drawing, reference numeral 1 denotes an antenna; 2, an RF amplifier; 3, a frequency converter (MIX); 4, a local oscillator (LO), 5, an intermediate frequency amplifier (IF AMP); 6, an orthogonal demodulator (DEMOD); 7, an A/D converter; 8, a synchronizing signal detector (synchronous detection); 9, a synchronization control means; 10, a complex discrete Fourier transform processing (hereafter referred to as xe2x80x9cDFTxe2x80x9d) means; 11, a differential demodulator; 12, a phase error detector; 13, a frequency tuning control means; 14, a Viterbi decoder; 15, an MPEG audio decoder; 16, a D/A converter; 17, an audio amplifier; and 18, a speaker.
In the receiver configured as described above, the broadcast wave received by the antenna 1 is amplified by the RF amplifier 2, is subjected to frequency conversion by the frequency converter 3, is subjected to removal of unwanted components such as adjacent channel waves and amplification by the intermediate frequency amplifier 5, is subjected to detection by the orthogonal demodulator 6, and is imparted to the A/D converter 7 as a baseband signal.
The signal sampled by the A/D converter 7 is subjected to DFT by the DFT means 10, and the phase of each transmission carrier subjected to quadrature phase shift keying (QPSK) is detected. In the ensuing differential demodulator 11, modulated phases of the same carrier of two transmitted symbols which are timewise adjacent to each other are compared, and processing (differential demodulation) for outputting a phase shift in the mean time is effected. The data subjected to differential demodulation is then outputted to the Viterbi decoder 14 in accordance with a rule on the order of carriers used in modulation on the transmitting side.
In the Viterbi decoder 14, interleaving is canceled during the time spanning over the range of a plurality of symbols transmitted by the transmitting side, the data transmitted through convolutional coding is decoded, and correction of errors of data occurring on the transmission path is effected at that time.
In accordance with the provisions of the layer-2 of ISO/MPEG1, the MPEG audio decoder 15 expands the compressed DAB broadcast audio data outputted from the Viterbi decoder 14, and sends the same to the D/A converter 16. The audio signal subjected to analog conversion by the D/A converter 16 is reproduced by the speaker 18 via the amplifier 17.
Here, the synchronizing signal detector 8 detects the null symbol (the period during which no signal is present) by envelope detection, in the frame alignment signal included in the transmitted signal of DAB. This output serves as a timing signal by which DFT effected by the DFT means 10 through the synchronization control means 9 is executed correctly in synchronism with the transmission frame and each symbol of the signal.
The phase error detector 12 detects an error between an original phase point and the phase data of each carrier outputted from the differential demodulator 11. That is, in DAB, if the frequency of the signal imparted to the orthogonal demodulator 6 is correct, the phase of the differentially demodulated data outputted from the differential demodulator 11 in correspondence with each carrier becomes substantially one of xcfx80/4, 3xc2x7xcfx80/4, 5xc2x7xcfx80/4, and 7xc2x7xcfx80/4.
Accordingly, if the data corresponding to each carrier is multiplied by 4 and the remainder is obtained with respect to 2xcfx80, this value becomes xcfx80 if there is no error in the original data, and becomes a multiple of 4 of that value if there is a phase error in the original data, so that phase error detection is carried out. In practice, in the phase error detector 12, the aforementioned operation is performed with respect to the data of the multiplicity of carriers, and the accuracy of detection is improved by averaging the results.
Since the phase error xcex5 thus determined is an output from the differential demodulator 11, the relationship of the following Formula (1) holds between an error xcex6 of the signal frequency at this time and the phase error xcex5:
xcex6=xcex5/T xe2x80x83xe2x80x83(1)
Here, T is a symbol period including a guard interval.
The frequency tuning control means 13 operates in such a manner as to cause the frequency error xcex6 of the baseband signal imparted from the orthogonal demodulator 6 to approach 0 by controlling the frequency of the intermediate frequency signal outputted from the frequency converter 3 by controlling the frequency of the local oscillator 4 in such a manner that this phase error xcex5 becomes small.
As already described, the DAB signal is comprised of a multiplicity of carriers. To separate the carriers, DFT has an output characteristic shown in FIG. 17, and when the frequency is pulled in correctly, components from other carriers do not leak.
However, when the frequency is not pulled in correctly, components from other carriers leak, as shown in FIG. 18.
Here, if there is no leakage from other carriers even if there is a frequency deviation, adjacent carriers s1and s2can be expressed by the following Formula (2):
s1=exp{j(2xcfx80(f0+xcex94fxe2x88x92nxc2x7fcc)t}
s2=exp{j(2xcfx80(f0+xcex94fxe2x88x92nxc2x7fcc)
(t+tsym)+xcex8c+xcex8n)}xe2x80x83xe2x80x83(2)
where, f0: transmission frequency
xcex94f: frequency deviation
n: carrier number
fcc: interval between carrier frequencies
tsym: the period of one symbol
xcex8n: (2N+1)p/4, N is an arbitrary integer
xcex8c: 2xcfx80(f0xe2x88x92nxc2x7fcc)xc2x7tsym
Accordingly, the phase error from (2xc2x7.N+1)xcfx80/4 of the same carrier of adjacent symbols can be expressed by the following Formula (3):
xcex8=xcex94fxc2x7tsymxe2x80x83xe2x80x83(3)
Hence, it can be seen that the phase error is proportional to the frequency deviation.
In practice, however, when the frequency has deviated, if there is leakage from other carriers, e.g., a frequency of xe2x88x9280 Hz, large variations appear in the differential modulated data, as shown in FIG. 19. Here, the differentially demodulated data is divided into four quadrants of 0xe2x88x92xcfx80/2, xcfx80/2xe2x88x92xcfx80, xcfx80xe2x88x923xcfx80/2, and 3xcfx80/2xe2x88x922xcfx80, but there occurs data which enters adjacent quadrants as shown in FIG. 19, and the sign of the data which shifted to adjacent quadrants becomes opposite and such data constitutes a large phase error. Since erroneous data in which the sign of phase error is opposite is also used in averaging processing by the phase error detector, the detected phase error assumes a value smaller than a real value.
In addition, the greater the deviation of the frequency, the greater the leakage of components from other carriers, so that the variation becomes larger, and the data is located closer to the adjacent quadrants, with the result that the aforementioned error is liable to occur. For this reason, as for the frequency deviation and the average value of phase errors, the phase error becomes small starting from the frequency deviation of 70 Hz or thereabouts, where the frequency deviation and the average phase error cease to be proportional. For this reason, if the frequency deviation is large, there has been a problem in that it takes time in the pulling in of the frequency.
The present invention has been devised to overcome the above-described problem, and its object is to obtain a digital audio broadcasting receiver which is provided with a frequency control means for a local oscillator which is not affected by variations in the phase error due to the frequency deviation.
In the digital audio broadcasting receiver in accordance with the present invention, a DAB signal inputted from the antenna is subjected to OFDM demodulation by the DFT, the phase difference between two successive symbols on the same carrier is calculated by the differential demodulator, the deviation of the differentially demodulated data in an N-th quadrant from a (2Nxe2x88x921)xcfx80/4 radian is detected as the phase difference by the phase error detector, the phase errors of the carriers are averaged by the average value processing unit, the sign of the phase errors is detected by the sign determining unit, the phase error is corrected by excluding the effect of data which changed to an adjacent carrier by the phase error correcting unit in correspondence with the result of determination by the sign determining unit, and the frequency of the local oscillator is controlled by the corrected phase error.
In the digital audio broadcasting receiver in accordance with the present invention, a DAB signal inputted from the antenna is subjected to OFDM demodulation by the DFT, the phase difference between two successive symbols on the same carrier is calculated by the differential demodulator, the deviation of the differentially demodulated data in an N-th quadrant from a (2Nxe2x88x921)xcfx80/4 radian is detected as the phase difference by the phase error detector, the phase errors of the carriers are averaged by the average value processing unit, the sign of the phase errors is detected by the sign determining unit, and the frequency of the local oscillator is controlled by restoring the data which changed to an adjacent carrier by the phase error correcting unit in correspondence with the result of determination by the sign determining unit.
In addition, a DAB signal inputted from the antenna is subjected to OFDM demodulation by the DFT, the phase lo difference between two successive symbols on the same carrier is calculated by the differential demodulator, a phase rotation by a (2Nxe2x88x921)xcfx80/4 radian is imparted to the differentially demodulated data in an N-th quadrant by the phase rotating unit, the sign of imaginary parts of the data after the phase rotation is determined by the imaginary-part sign determining unit, addition is effected with respect to only the data whose signs of the imaginary parts are the same, the phase error detecting unit detects the phase error by excluding the effect of data which changed to an adjacent carrier, and the frequency of the local oscillator is controlled.
In addition, a DAB signal inputted from the antenna is subjected to OFDM demodulation by the DFT, the phase difference between two successive symbols on the same carrier is calculated by the differential demodulator, a phase rotation by a (2Nxe2x88x921)xcfx80/4 radian is imparted to the differentially demodulated data in an N-th quadrant by the phase rotating unit, the sign of imaginary parts of the data after the phase rotation is determined by the imaginary-part sign determining unit, addition is effected with respect to only the data whose signs of the imaginary parts are the same, the effect of data which changed to an adjacent carrier is restored by the phase error detecting unit, and the frequency of the local oscillator is controlled.
In addition, a DAB signal inputted from the antenna is subjected to OFDM demodulation by the DFT, the phase difference between two successive symbols on the same carrier is calculated by the differential demodulator, the deviation of the differentially demodulated data in an N-th quadrant from a (2Nxe2x88x921)xcfx80/4 radian is detected as the phase difference by the phase error detector, the phase errors of the carriers are averaged by the average value processing unit, the relative magnitude of leakage from another carrier is determined on the basis of output data from the differential demodulator, the average value of phase errors is corrected by the phase error correcting unit if the leakage from another carrier is large, and the frequency of the local oscillator is controlled.
In addition, a DAB signal inputted from the antenna is subjected to OFDM demodulation by the DFT, the phase difference between two successive symbols on the same carrier is calculated by the differential demodulator, the deviation of the differentially demodulated data in an N-th quadrant from a (2Nxe2x88x921)xcfx80/4 radian is detected as the phase difference by the phase error detector, the phase errors of the carriers are averaged by the average value processing unit, the variation of the differentially demodulated data is detected by the variation determining unit, the average value of phase errors is corrected if the variation is large, and the frequency of the local oscillator is controlled.
In addition, a DAB signal inputted from the antenna is subjected to OFDM demodulation by the DFT, the phase difference between two successive symbols on the same carrier is calculated by the differential demodulator, the deviation of the differentially demodulated data in an N-th quadrant from a (2Nxe2x88x921)xcfx80/4 radian is detected as the phase difference by the phase error detector, the phase errors of the carriers are averaged by the average value processing unit, an inclination of the magnitude of the phase error is detected by the inclination detecting unit, the average value of phase errors is corrected if the inclination is not in a converging direction, and the frequency of the local oscillator is controlled by the corrected phase error.
In the digital audio broadcasting receiver in accordance with the present invention, the phase error correcting unit handles the phase error whose sign is different from the sign of the average value of phase errors as being data in an adjacent quadrant since the phase difference in the differentially demodulated data has exceeded xc2x1xcfx80/2, and determines that the data is erroneous. Hence, the phase error correcting unit corrects the phase error by effecting averaging with respect to only the phase errors whose sign agrees with the sign of the average value of phase errors.
In addition, in the digital audio broadcasting receiver in accordance with the present invention, in the restoration of the phase error by the phase error correcting unit, if the sign of the phase error is different from the that of the average value, the phase error is considered as being data in an adjacent quadrant since the phase difference in the differentially demodulated data has exceeded xc2x1xcfx80/2. If the phase error is assumed to be q, the phase error correcting unit effects correction of xcex8xe2x88x92xcfx80/2 if the phase error is plus, and xcex8+xcfx80/2 if the phase error is minus.
In addition, in the digital audio broadcasting receiver in accordance with the present invention, if the absolute value is smaller between the absolute value of the sum of plus imaginary parts and the absolute value of the sum of minus imaginary parts, the phase error is xc2x1xcfx80/2 or more, so that such data is handled as being data in an adjacent quadrant. Hence, since it is considered that the sign has been erroneous, the phase error correcting unit calculates imaginary parts/real parts of only the data whose absolute value is greater, and outputs the same as the phase error.
In addition, in the digital audio broadcasting receiver in accordance with the present invention, the phase error correcting unit calculates imaginary parts/real parts for plus imaginary parts and imaginary parts/real parts for minus imaginary parts, and the data which exhibits a greater absolute value between plus imaginary parts and minus imaginary parts is left as it is and is set as a phase error 1. Meanwhile, in the case of data which exhibits a smaller absolute value is handled as data in an adjacent quadrant since the phase error has exceeded xc2x1xcfx80/2, and it is considered that the sign of such data has been erroneous. Accordingly, if the phase error is assumed to be q, the phase error correcting unit effects correction of xc2x1xe2x88x92xcfx80/2 if the imaginary parts are plus, and xcex8+xcfx80/2 if the imaginary parts are minus, thereby restoring the phase error to an original phase error as a phase error 2. An average of the phase error 1 and the phase error 2 is used as the phase error.
In addition, in the digital audio broadcasting receiver in accordance with the present invention, if the leakage from another carrier becomes large, the phase error becomes smaller than a real value due to the leakage from another carrier, so that the phase error correcting unit provides processing for increasing the phase error, for example.
In addition, in the digital audio broadcasting receiver in accordance with the present invention, the greater the frequency deviation, the more the leakage from another carrier increases in the result of DFT, and the phase is also affected. The effect becomes large when the phase difference with respect to a neighboring carrier is xc2x1xcfx80/2, but the phase difference with a neighboring carrier is not uniform. For this reason, the leakage from other carriers also changes. Therefore, variations occur in the result of differential modulation as shown in FIG. 19. The greater the leakage from other carriers, the larger the variations, so that the variation of the data is calculated by the phase error correcting unit to determine the relative magnitude of leakage from other carriers.
In addition, in the digital audio broadcasting receiver in accordance with the present invention, the leakage component makes use of the fact that, between the region where the leakage from another carrier is large and the region where it is small, the signs of inclination of phase errors with respect to the frequency deviation are opposite. First, since feedback is provided to the local oscillator in such a manner that the phase difference approaches 0, the magnitude of the phase difference becomes small in the region where the leakage from other carriers is small. However, in the region where the frequency leakage is large, even if the phase error is small, the effect of leakage components from other carriers becomes small and the phase error approaches a real phase error, so that the phase error becomes apparently large. Accordingly, the leakage from other carriers is detected by performing a calculation in accordance with the following formula:
xcex94xcex8= (absolute value of the average value of current phase errors)xe2x88x92(absolute value of the average value of previous phase errors) If the sign of the previous errors and the sign of the current phase errors are the same, it is considered that the real phase error is approaching 0. As a result, if xcex94xcex8 is plus, it can be determined that the phase error is becoming smaller than the real value due to the leakage from other carriers.